The disclosure relates generally to an assembly having, and a method providing, improvement in thermal oxidation and corona discharge resistance, and more particularly to an assembly and a method for forming a flexible high heat resistant dielectric material.
Polymeric films are known to have utility as insulating materials in motors and generators. Known polymeric films serve as dielectric materials insulating conducting materials from other conducting materials to inhibit shorting, or short circuiting, of an electrical connection. Insulation provides protection against voltage hazards and inhibits leakage of current as well as electric discharge and short circuits.
FIG. 1 illustrates schematically a section of a motor 10. Polymeric films are used as insulating materials in various locations. For example, polymeric films are used as phase insulation/end winding insulation 12. Also, polymeric films are used as ground insulation/slot liners 14. Polymeric films can also be used as turn insulation 16, a copper wire coating. Wound wires 18, 20 and 22 are positioned relative to a voltage stress level in the motor 10. For an AC motor or generator, usually there are three voltage phases, 120 degree apart. Wound wire 18 generally refers to wires next to each other in two different phases, where it has the highest voltage drop, so insulator in addition to wire coating is needed to separate these phase-to-phase voltage drops. The wound wire 20 generally refers to a wire next to the steel core (or steel laminates) which is grounded. The voltage between the wire 20 and the core is the line voltage to ground which is also high, so ground insulation in addition to wire coating is needed. Wound wire 22 refers to wires next to each other in the same voltage phase, where the voltage drop is the least, so the coating on the conductive wire may provide sufficient insulation.
The polymeric films that are currently used in motors and generators are formed of one or more of cross-linked polyethylene, polypropylene, polyester, polycarbonate, polyurethane, polyphenylene oxide, high heat polymer films such as polyimide, aromatic polyimide, aromatic polyester, polyetherimide, polyamideimide, polyphenylene sulfide, polyphenylene sulfone, polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE) and other fluoropolymers.
Film-like material is often used with conducting materials like wires used in electrical machines because such material lends flexibility. Flexibility is needed in that the conducting materials often are wound or are maintained in curved and/or non-planar orientations. To properly coat such conducting materials without creating undue stress on the conducting materials, thin film-like materials are used. Film-like materials are also frequently used as phase separation and slot liners for winding wires. Flexibility and abrasion resistance of films are needed for them to survive the mechanical stress during manufacturing assembly processes.
However, disadvantages exist in known polymeric films used to insulate conducting materials within motors and/or generators. Currently known polymeric films have heat stability or thermal index only up to 260° C. What is meant by “heat stability” or “thermal index” is that the material's dielectric and/or mechanical integrity is intact after 20,000 hours of thermal aging at 260° C. The standard test method used for defining thermal index can be found in ASTM D2307. Newer motors and generators require materials which can withstand higher heat than 260° C., and therefore often are manufactured to operate at higher temperatures.
Previous generation electric drives mostly operated with line voltage operated at a constant frequency unlike newer pulse-width modulated (PWM) driven motor/drives driven by high dV/dT PWM drives and operated near or higher than Partial Discharge Inception Voltages (PDIV) or corona inception voltage (CIV).
In addition, limited volume/space limits the separation and spacing of high voltage signals/power lines in electric machine windings as well as cabling and power electronics combined with low pressure with high temperature often results in the operation near or higher than PDIV/CIV for electric discharge.
Where polymeric film is used in high temperature applications, mica, ceramic and glass tape have been traditionally employed to provide greater heat resistance. However, because of their rigidity and low dielectric strength, high thickness is required to achieve proper dielectric strength. The size and weight of power units utilizing these types of insulation tends to be large and heavy, respectively, and thus power density of the system as a whole is sacrificed.
Another disadvantage is that known polymeric films can only withstand corona discharges for a limited period of time. For example, in an experiment run by the inventors a polyimide film was exposed to a 20 kilohertz (kHz) continuous square wave pulse. The polyimide film lasted less than 10 minutes without degrading to the point of breaking down, or short circuiting.
Given the known disadvantages of the current state of the art of insulating films, an improved insulation assembly and method for insulating conducting materials in an electrical machine would be welcome in the art.